The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper chamber to the lower chamber necessary for performing normal systole and diastole function. The presence of an arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to herein as tachycardias.
A variety of approaches, including drugs, implantable pacemakers/defibrillators, surgery, and catheter ablation have been proposed to treat tachycardias. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter tip to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within or around the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablations tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of the arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated. However, in the case of atrial fibrillation (AFib) or atrial flutter, multiple arrhythmogenic sites and/or multiple accessory pathways exist. The conventional catheter with a single "stationary" ablation electrode can not effectively cure the symptoms. In the case of paroxysmal atrial fibrillation, a circular lesion at about the pulmonary vein is required.
Atrial fibrillation is believed to be the result of the simultaneous occurrence of multiple wavelets of functional re-entry of electrical impulses within the atria, resulting in a condition in which the transmission of electrical activity becomes so disorganized that the atria contracts irregularly. Once considered a benign disorder, AFib now is widely recognized as the cause of significant morbidity and mortality. The most dangerous outcome from AFib is thromboembolism and stroke risk, the latter due to the chaotic contractions of the atria causing blood to pool. This in turn can lead to clot formation and the potential for an embolic stroke. According to data from the American Heart Association, about 75,000 strokes per year are AFib-related.
A catheter utilized in the endocardial RF ablation is inserted into a major vein or artery, usually in the neck or groin area. For paroxysmal AFib indications, a catheter is approached from the atrium to the ostium of a pulmonary vein. The tip section of a catheter is referred to here as the portion of that catheter shaft containing means for thermal lesion which may be deflectable and may be adapted to form a circular or an irregular-shape loop lesion. The means for circular thermal lesion is to be positioned against the ostium of the pulmonary vein, whereby the circular ablation means having a firm element, such as a heated inflatable balloon, can be pressed against the tissue for circular ablation.
The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large electrode about 4 to 8 mm in length for ablation purposes. Sometimes, a plurality of long electrodes is used in creating a contiguous linear lesion. However, for creating a circular lesion with uniform lesion quality, the temperatures at various device-to-tissue contact sites should be as uniform as possible. A catheter with a heated balloon has been widely used to apply heat to the balloon-to-tissue sites. The temperature of the fluid medium inside an inflated balloon is far from uniform because the heat source and fluid medium are stationary having poor heat transfer property.
Avitall in the U.S. Pat. No. 5,242,441 teaches a rotatable tip electrode. Said electrode is secured to a high torque wire for rotation and electrical conductivity. The tissue contact site is always the same spot even the electrode is rotated. Moreover, a movable band electrode has been recently introduced to the market to simulate the "rollable electrode" concept. Since the said band electrode does not roll, the contact surface spot of the said band electrode with tissues is always the same spot. The potential coagulum at the contact electrode surface spot due to impedance and temperature rises, would not go away because of its relatively stationary position of the rotatable tip electrode or the movable band electrode.
U.S. Pat. No. 5,840,076 discloses a balloon type electrode catheter by using balloon as a medium to create a circular lesion, wherein the balloon is made of a porous material. Said patent discloses a RF circuit by including a patient in the circuit loop, whereby the heat generated by the uni-polar RF current at the tissue contact site may unexpectedly hurt the patient. The local fixed heat source of the uni-polar means may make the temperature of the fluid inside and around the heated balloon non-uniform. The temperature at the heat source is highest whereas the heat can spread to different regions of the fluid of the inflated balloon when a movable heat source is used.
While a radiofrequency electrophysiology ablation procedure using an existing balloon catheter has had promising results, the temperature at the balloon periphery should be uniform so that a uniform circular lesion can be created for the paroxysmal AFib at a pulmonary vein. Therefore there is a need for an improved catheter and methods for making a circular lesion in the cardiac tissue employing a balloon-type thermal ablation with a bipolar electrode pair means within an inflated balloon.